Network TV erosion is like an overused sports metaphor, say for LeBron James or Kobe Bryant: "You can't stop it, you can only hope to contain it." (More sports metaphors to follow, in parentheses.)This season witnessed a 16% drop in network ratings, following a double-digit decline a year ago -- the year of the writer's strike. (It's going to be a long plane ride home.)But that's not the real problem: With erosion moving at a double-digit pace - versus the mid-single digits of recent years - when do TV advertisers try a new game plan? (Turnovers killed us.)Say there is a 15% decline in the 2009-2010 season, and then a 12% decline in the 2010-2011 season? That would result in broadcast ratings sinking a collective 30% -- on top of a 30% drop over the last two and a half years. (He's lost a step or two.)At some point there will be a major crack in the TV universe, with money going elsewhere -- cable, digital video, sandwich boards, or coupons. (They're in a must-win situation.)You can talk about cable all your want, as well as the still-slow rise in TV usage overall. But at some point established TV cable networks will no doubt witness some viewer erosion of their own, as newer technologies and more fractionalization occurs. (He's been the subject of trade rumors).In three years time, that means the average three rating among 18-49 viewers of network shows, will be a two rating. By then "American Idol" will be doing below 20 million total viewers only once a year -- at its finale. (Not to take away anything from Denver, but we didn't play like we're capable of playing.)The only big TV stuff left to buy will be the Super Bowl and maybe the Academy Awards. All this will mean is: we need to take it one game at a time. Or, maybe it's gut-check time.